1. Technical Field
The present invention relates generally to a control system for controlling a traveling condition of an automotive vehicle. More particularly, the invention relates to a traction control system which controls engine and braking operations to modify driving forces transmitted to driven wheels upon an occurrence of wheel slippage during acceleration.
2. Background Art
An acceleration-caused slippage control system is well known in the art which controls wheel slippage when an automotive vehicle starts quickly and/or during acceleration. This slippage control system determines that acceleration-caused wheel slippage occurs, for example, when a slip ratio of driven wheels exceeds a reference slip ratio, and controls the acceleration-caused wheel slippage so that the slip ratio of the driven wheels may fall within an allowable range.
Specifically, when wheel slippage occurs during acceleration, the system performs engine control which regulates an opening degree of a throttle valve based on the average speeds of right and left driven wheels to control the engine power, and braking control which adjusts the pressure of brake fluid for modifying braking forces applied to the driven wheels.
The above slippage control system, however, encounters the following drawback. When a slip ratio exceeds a threshold value, the system determines that wheel slippage has occurred, and performs engine control and the braking control simultaneously. Thus, for example, if a distribution ratio between the engine control and the braking control is set suitably for controlling wheel slippage in a low-middle speed range, excessive braking forces may be applied to the driven wheels in relation to a gear ratio according to vehicle speed, thereby causing undesirable deceleration, or braking forces applied to the right and left driven wheels to be put out of balance. This results in a deterioration in straight line stability.
For avoiding the above drawback, Japanese Patent First Publication No. 3-74226 teaches a traction control system which performs engine control when a slip ratio reaches an engine control threshold value smaller than a braking control threshold value at which braking control is initiated in a high speed range, in other words, a ratio of the braking control to the engine control under acceleration-caused slippage control is decreased.
However, since the above prior art system sets the two threshold values only in the high speed range, it there still is room for improvement in achieving more stable traveling conditions.
For example, when the vehicle accelerates from the lower speed range to the high speed range, the engine control threshold value is changed suddenly, causing control conditions to be changed suddenly. This is quite inconvenient for stable control of vehicle traveling conditions.